PET-CT scan
Also known as: positron emission tomography, PET scan, FDG-PET, nuclear medicine scan, bone scan, SPECT
Positron emission tomography tracks a mildly radioactive sugar or tracer to show where cells are most active, usually combined with CT to stage cancer and check treatment response.
How it works
A tiny amount of a radioactive tracer is injected and the scanner detects where it accumulates. The commonest tracer, FDG, is a glucose analogue, so tissues that burn a lot of sugar (most cancers, infection, inflammation, the brain and heart) light up. The PET image is fused with a CT taken at the same time to show exactly where the activity sits. Other nuclear medicine scans use tracers that go to bone, the thyroid, the heart muscle or the kidneys.
What it shows
- Cancer: staging, finding spread, judging response to treatment, distinguishing scar from active tumour
- Infection and inflammation of unknown source, vasculitis, infected grafts
- Brain: dementia patterns, epilepsy foci
- Heart: viable versus scarred muscle
- Bone scan: metastases, stress fractures, infection, Paget disease
- Thyroid uptake scan: overactive nodules versus thyroiditis
Best for
- Whole-body cancer staging in lung cancer, lymphoma, melanoma, oesophageal and head and neck cancer
- Deciding whether a lung nodule is likely malignant
- Fever of unknown origin
Not the right tool for
- Cancers with low glucose uptake (some prostate, kidney, low-grade tumours; other tracers are used)
- Small lesions under about 5 mm
- Brain metastases (the brain glows with normal uptake; MRI instead)
- Uncontrolled diabetes, which spoils FDG uptake
What to expect
- Modality
- PET (positron emission tomography)
- Preparation
- Fast for 6 hours, avoid exercise for 24 hours, keep warm; blood glucose checked. Sit quietly for an hour after injection while the tracer distributes.
- Duration
- About 2-3 hours in the department; scanning 20-30 minutes.
- Radiation
- Around 7 mSv from the tracer plus the CT component, in total roughly 10-25 mSv depending on protocol.
- Contrast
- The radiotracer itself; the CT component may be low-dose without contrast or a full diagnostic scan with iodine.
- Risks
- Radiation as above; avoid close contact with pregnant women and small children for a few hours after the tracer. No allergic risk from FDG.
Common uses
| Region | Typical question |
|---|---|
| Whole body | Staging and restaging cancer, planning radiotherapy |
| Chest | Characterising a lung nodule; mediastinal lymph nodes |
| Brain | Frontotemporal versus Alzheimer dementia; amyloid PET |
| Bone | Bone scan for metastases or occult fracture |
Studies this page covers
Named studies of the master test taxonomy that this modality page covers; each links to its test category.